Potentiometric sensors are used for detecting chemical or biochemical compounds in a solution. Such potentiometric sensors are electrochemical sensors that generate voltages that scale with the concentration of an ion to be determined. A pH sensor is an example of such a potentiometric sensor. A potentiometric sensor usually comprises an ion-selective electrode and a reference electrode. The reference electrode has a potential which can be used as a reference potential for the ion-selective electrode. The potential difference between the ion-selective electrode and the reference electrode is a measure for the concentration of the compound for which the ion-selective electrode is sensitive. An important requirement is that the potential of the reference electrode is stable and constant.
In amperometric sensors, the sensing electrode is biased at a fixed potential versus the reference electrode. At this potential, ions or molecules of interest are reduced or oxidized at the sensing electrode. By measuring the reducing or oxidizing current at the sensing electrode, the concentration of ions or molecules of interest can be determined. Also here, the stability of the reference electrode potential is crucial for the sensitivity of the sensor.
In operation, the reference electrode is immersed, together with the ion-selective electrode, in the solution under test. For proper operation, the interfacial potential of the reference electrode should be independent of the solution composition.
A commonly used type of reference electrode is a silver chloride electrode (Ag/AgCl). This electrode has a fixed potential when in contact with a reservoir with a fixed chloride concentration (e.g. 3 Molar KCl). An example of such conventional reference electrode is shown in FIG. 1. FIG. 1 shows a conventional reference electrode comprising an Ag/AgCl wire 110, immersed in a reservoir 120 containing a predetermined chloride concentration, for instance 3 Molar KCl (the reference electrolyte solution). An electrolyte bridge or a porous ceramic plug separates the reference electrolyte solution from the solution which needs to be measured. In the exemplary reference electrode of FIG. 1 the electrolyte bridge is a porous frit (salt bridge) 130 separating the inner reservoir 120 from the bulk solution. Ions can still pass through this junction; therefore an ionic contact forms. Ion selective electrodes also use an internal electrolyte. In that case, instead of a frit, an ion selective membrane separates the internal electrolyte from the solution which needs to be measured. The potential of the Ag/AgCl electrode depends on the chloride concentration in the reference electrolyte solution. The electrolyte bridge or the porous ceramic plug 130 prevents that chloride ions instantaneously migrate between the liquid of the reference electrode and the bulk solution. If the reference electrode is immersed in a solution with different chloride concentration, chloride ions will leach through the electrolyte bridge. Depending on the volume of the reference electrolyte solution these migrations will cause the chloride concentration in the reference electrolyte solution to change, thereby changing the reference voltage (drift). As the reservoir is rather large in the example of FIG. 1, drift is limited. One way of stabilizing the reference voltage is to increase the volume of the reference electrolyte solution. This is, however, not possible in microfabricated reference electrodes since the size is, per definition, limited in these electrodes.
Microfabricated reference electrodes typically comprise a planar electrode, made by photolithography or screen-printing techniques covered by a hydrogel such as agarose or polyhydroxyethylmethacrylate (e.g. Simonis et. al. “Miniaturized reference electrodes for field-effect sensors compatible to silicon chip technology”, Electrochimica Acta 51, Volume 51, issue 5, 10 Nov. 2005, pages 930-937).
Increasing the stability of miniaturized reference electrodes can be done without increasing the volume of the reference electrolyte solution. Many studies on miniaturized reference electrodes have focused on the composition of the reference electrolyte solution (e.g. a hydrogel), and covering it with membranes to slow down the out diffusion of chloride ions. Besides these efforts there is still room for improving the stability of miniaturized reference electrodes.